Aorto-caval fistula

Clinical History

A 72-year-old male patient with a history of hypertension was admitted with syncope as chief complaint. On initial examination his blood pressure was 143/92 mmHg. Three days later, a palpable pulsatile abdominal mass was noted. The patient became haemodynamically unstable and passed away minutes after the accurate diagnosis was established.

Imaging Findings

A contrast-enhanced CT was performed in the arterial phase where a 10 x 14 cm infrarenal abdominal aortic aneurysm (AAA) and a fistula between the aorta and the inferior vena cava (IVC) was noticed.
The images displayed simultaneous contrast enhancement of the AAA and the adjacent IVC, which is the main characteristic CT findings of an aorto-caval fistula (ACF). [1]
No free fluid was present in the retroperitoneal region or in the rest of the abdomen, indicating no acute rupture at the time of the CT examination.

Discussion

Aorto-caval fistula (ACF) occurs in <1% of all abdominal aortic aneurysms (AAA), and in 3-7% of all ruptured aneurysms, which is the most common cause of ACF. [2] The most common rupture in AAA occurs into the retroperitoneal space (95%). [3]
Atherosclerosis is the main reason for AAA which can also develop due to trauma, spine surgery and connective tissue disorders. [4]

The classic triade of AAA is 1) Abdominal and/or back pain. 2) A pulsatile mass (most common) 3) An abdominal machinery bruit. ACF can be seen with or without retroperitoneal rupture, and therefore mimic other cardiovascular conditions that cause lower limb oedema and congestive heart failure. ACF does not necessarily present with the classic triade for AAA. Clinical manifestations such as haematuria, oliguria, anuria, dyspnoea, syncope, haemorrhagic shock, renal insufficiency, palpitations and elevated central venous pressure has been associated with ACF. In this case, the patient was admitted two months prior with a similar complaint. Due to a history of apoplexia cerebri, the patient was admitted to the Dept. of Neurology. A palpation of the abdomen and a CT scan was not performed until the patient presented abdominal symptoms.

The gold standard imaging method for an early establishment of AAA is Computed Tomography (CT) with intravenous contrast. [2] Imaging pitfalls include false lumen thrombus in aortic dissection, intramural haematoma and collapse of adjacent lung parenchyma. A low haematocrit in anaemic patients may cause the arterial wall to appear hyperattenuated on unenhanced CT. On post contrast CT, technical factors such as slow contrast injection may determine a missed recognition of the ACF. [1]

The diagnosis is confirmed preoperatively in the majority of cases. With treatment, mortality rate is 25-50% (overall 67%) in haemodynamically unstable patients. [5] In elective surgery, with stable patients, the mortality rate has been reported to be as low as 7.1 %. Without surgery, the condition is fatal within two months. [6] Treatment is either conventional open repair or a hybrid approach with endovascular repair.

Familiarity with the characteristic imaging features of AAA and ACF is essential for radiologists. Furthermore, the referring physician’s awareness of this clinically rare entity and the importance of a full body examination when admitting a patient are the most important factors for the prompt diagnosis of this potentially lethal condition.
Also, without i.v. contrast, an ACF will be overseen, thus the importance of using contrast regardless of impaired renal function since ACF is fatal without treatment.

Differential Diagnosis List

Final Diagnosis

Abdominal Aortic Aneurysm with Aorto-Caval Fistula

URL:	https://www.eurorad.org/case/13172
DOI:	10.1594/EURORAD/CASE.13172
ISSN:	1563-4086